1. Field of the Invention
This invention relates generally to rotating data storage devices and specifically to rotating magnetic disks having controlled surface topology for reducing magnetic head contact and dwell stiction upon startup and stopping of disk rotation and improving disk durability.
2. Description of the Related Art
Modern data storage devices include magnetic disk drives such as the Direct Access Storage Device (DASD) employing one or more disks rotated on a central axis in combination with magnetic heads positioned closely to the disk recording surfaces for transducing recorded magnetic signals. Recent magnetic disk drive designs employ a commonly denominated Contact Start-Stop (CSS) system in which the magnetic head is in contact with the magnetic disk surface when the disk is stationary. When the disk starts to rotate, the magnetic head slides along the surface, eventually flying fully lifted from the surface because of laminar air flow at the disk surface.
A smooth, specular recording surface is preferred in the art to permit the magnetic head to ride as closely as possible to the disk surface. However, for extremely smooth disk and head air-bearing surfaces, excessive contact stiction and friction is developed during startup and stopping of disk rotation, resulting in the need for higher torque motors, disk wear and eventual disk failure. The possible solution known in the art for this "stiction" problem is to reserve a "textured" region of the rotating disk surface for magnetic head contact during CSS. The surface texture in the "CSS region" reduces the contact stiction and friction. The magnetic head is moved to the CSS region at the appropriate times by the drive controller. The specular smoothness of the remainder of the disk surface is retained to permit high-density magnetic data recording.
Until recently, texturizing of the CSS region of the disk surface was usually accomplished by contact abrasion means using mechanical abrasive pads or powders to increase surface "roughness" in a random manner (see FIG. 1A). Chemical etching methods are also known for use with special disk substrate materials, including nonmetals. These mechanical and chemical techniques are relatively uncontrollable and expensive, lacking precision, tool reliability and speed. Because CSS region texturing is needed in modern and future disk drives for stiction reduction, there has long been a clearly-felt need for improved CSS region texturing techniques and materials.
In U.S. Pat. No. 4,626,941, Shigetomo Sawada et al. disclose a disk texturing technique designed to suppress lubricant evaporation and enhance lubricant adhesion to the rotating disk surface. Sawada et al. suggest sandblasting, machining and like methods for roughening of the outer disk surface.
In U.S. Pat. No. 5,008,687, Robert J. Longman et al. disclose a data storage medium having a textured surface pattern consisting of an arrangement of grooves or protuberances, the pitch of which is smaller than the shortest wavelength within the electromagnetic radiation bands employed for magnetic data storage. Longman et al. teach the use of their sub-micron surface textures to enhance signal-to-noise ratios (SNRs) in certain alloy compositions used for magnetic data storage. They neither consider nor suggest the application of their textures to the magnetic head stiction problem in CSS disk drives.
In U.S. Pat. No. 4,698,251, Yasutaka Fukuda et al. disclose the use of concentric scratch marks formed on the surface of the disk to create magnetic anisotropy in the magnetic surface layer deposited on the scratched surface. Fukuda et al. use the mechanical grooves to force domain anisotropy in the later-deposited magnetic layer in a well-known manner and neither consider nor suggest the application of their mechanical scratching technique to the magnetic head stiction problem in CSS disk drives.
Several practitioners have suggested improved texturing techniques for reducing magnetic head stiction in CSS applications. For instance, in U.S. Pat. No. 5,070,425, Mitsuo Inumochi discloses a disk surface grooving technique designed to minimize head stiction over the entire surface. Inumochi teaches the use of two groups of streaks or grooves, with the streaks in one group crossing those in the second group at a controlled angle, thereby reducing the dynamic friction between the magnetic head and the magnetic disk. Inumochi doesn't discuss the serious problem of data storage errors arising from the disturbed surface topography at the boundaries of each of his grooves and offers no assurances that his invention is workable in combination with high data storage densities.
In U.S. Pat. Nos. 5,062,021 and 5,108,781, Rajiv Y. Ranjan et al. disclose a process for manufacturing selectively textured magnetic recording media employing a flashlamp-pumped neodymium-yttrium-aluminum-garnet (Nd:YAG) laser of the type well-known for use in laser trimming systems. Ranjan et al. teach the use of a series of closely-spaced craters to increase the disk surface roughness over a CSS region (see FIGS. 1B-1D). Their laser pulse frequency in combination with the disk rotational speed controls the pattern of overlapping laser craters in the disk surface. Although Ranjan et al. show that the coefficient of friction between the magnetic head and a disk surface textured in accordance with the method of their invention remains low over many thousands of CSS cycles, the surface texture created by the method of their invention (see FIGS. 1E-1F) is too rough to sustain high-density magnetic data storage. Moreover, the individual closely-spaced craters on the disk surface are not uniform (FIG. 1D) because of overlap and the controllability limits of the laser texturing method of their invention. Also, Ranjan et al consider only the crater-type profile shown in FIGS. 1B-1D and neither teach nor suggest creation of other useful profiles.
Accordingly, there is a clearly-felt need in the art for an improved disk texturing technique without process disadvantages such as limited controllability and loss of high-density data storage capacity in the textured area. Also, a crater-shaped dimple with a relatively rough rim is less desirable in terms of durability, flyability, and dwell stiction control when compared to bumps having a smooth central dome as the highest topographical feature. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.